The present invention generally relates to a transmission case.
In general, an automatic transmission mounted on, for example, a vehicle is structured by a hydraulic transmission apparatus such as a torque converter as a starting device, and a speed change mechanism formed by a gear mechanism such as planetary gears, a hydraulic clutch, and a hydraulic brake. The hydraulic transmission apparatus and the speed change mechanism are accommodated in a converter housing and a speed change mechanism case, respectively.
Automatic transmission fluid (ATF) is dispersed and circulated in the speed change mechanism case in order to lubricate and cool each portion of the speed change mechanism. The inside of the speed change mechanism case is therefore a space that is oil-tight to the outside of the speed change mechanism case. The converter housing, on the other hand, is a structural member that covers the outside of the hydraulic transmission apparatus and connects the automatic transmission to an engine. The space in the speed change mechanism case and the space in the converter housing are normally separated from each other, and a pump body of an oil pump as a hydraulic source for hydraulically controlling and lubricating each portion serves as a partition wall.
As a transmission case formed by the above-described speed change mechanism case and converter housing, a transmission case that prevents breakage of the speed change mechanism case and thus prevents oil leakage in a case where an offset impact load is applied to the transmission case mounted on, for example, an FR (front engine, rear drive) vehicle (such that the axial direction of an automatic transmission becomes parallel to the forward traveling direction of the vehicle) has been proposed (see Japanese Patent Application Publication No. JP-A-2003-336727).
In this transmission case, a slit (through hole) serving as a stress concentration portion is formed in the converter housing. In the case where an offset impact load is applied to the above-described so-called longitudinally-mounted automatic transmission, a broken part is limited to the converter housing, whereby breakage of the speed change mechanism case is prevented and oil leakage is prevented.